1. Field of the Invention
The present invention generally relates to cobalt alloy fabrication and, in particular, relates to the formulation of cobalt alloy matrix compositions which provide improved sputtering properties and enhanced distribution of major alloy elements, in order to improve the sputtering process and provide increased performance from the resulting thin film.
2. Description of the Related Art
The process of DC magnetron sputtering is widely used in a variety of fields to provide thin film material deposition of a precisely controlled thickness and within narrow atomic fraction tolerances on a substrate, for example to coat semiconductors and/or to form films on surfaces of magnetic recording media. In one common configuration, a racetrack-shaped magnetic field is applied to the sputter target by placing magnets on the backside surface of the target. Electrons are trapped near the sputter target, improving argon ion production and increasing the sputtering rate. Ions within this plasma collide with a surface of the sputter target causing the sputter target to emit atoms from the sputter target surface. The voltage difference between the cathodic sputter target and an anodic substrate that is to be coated causes the emitted atoms to form the desired film on the surface of the substrate.
During the production of conventional magnetic recording media, layers of thin films are sequentially sputtered onto a substrate by multiple sputter targets, where each sputter target is comprised of a different material, resulting in the deposition of a thin film “stack.” FIG. 1 illustrates one such typical thin film stack for conventional magnetic recording media. At the base of the stack is non-magnetic substrate 101, which is typically aluminum or glass. Seed layer 102, the first deposited layer, forces the shape and orientation of the grain structure of higher layers, and is commonly comprised of NiP or NiAl. Next, soft magnetic underlayer (“SUL”) 104, which is often comprised of alloys such as FeCoB, CoNbZr, CoTaZr or CoTaNb, is formed provide a return path for the read/write magnetic field. SUL 104 is amorphous, preventing magnetic domain formation which could potentially cause signal-to-noise (“SNR”) degradation.
Seed layer 105 is formed above SUL 104 in order to promote oriented growth of higher layers. Seed layer 105 is often comprised of ruthenium (Ru), since ruthenium (Ru) provides a hexagonal-close-packed (“HCP”) lattice parameter which is similar to cobalt (Co) HCP. For high-density data recording applications, magnetic data-storing layer 106 is deposited over seed layer 105, where data-storing layer 106 is a metal matrix composite composed of a ferromagnetic alloy matrix and a metal oxide. Typically, the ferromagnetic alloy matrix is typically a binary matrix alloy, such as CoPt, a ternary matrix alloy, such as CoCrPt, or a quaternary matrix alloy, such as CoCrPtX, where X is a boron (B), tantalum (Ta), niobium (Nb), zirconium (Zr), copper (Cu), silver (Ag) or gold (Au) alloy. Although many different oxides can be used, the most common metal oxide is either SiO2 or TiO2, due to the high affinity of base metals silicon (Si) and titanium (Ti) for oxygen, and observed beneficial data storage performance resulting from these oxides. Finally, carbon lubricant layer 108 is formed over magnetic data-storing layer 106.
The amount of data that can be stored per unit area on a magnetic recording medium is inversely proportional to the grain size of magnetic data-storing layer 106 and, correspondingly, to the sputter target material composition from which the data-storing layer is sputtered, where a ‘grain’ corresponds to a single, approximately ten nanometer crystal of the thin film alloy. Grain boundary segregation, a measure of the physical separation of the grains, also contributes to increased data storage capacity, where grain size and grain boundary segregation are directly influenced by the characteristics of the sputtering target microstructure from which the data-storing layer was sputtered, and the degree of structural refinement of the seed layer.
To sustain the continuous growth in data storage capacity demanded by magnetic data storage industry, a technique known as “perpendicular magnetic recording” (“PMR”), as opposed to conventional “longitudinal magnetic recording” (“LMR”), has been the most promising and efficient technology, due to its higher write efficiency using a perpendicular single-pole recording head, in combination with a soft underlayer. Using PMR, bits are recorded perpendicular to the plane of the magnetic recording medium, allowing for a smaller bit size and greater coercivity. In the future, PMR is expected to increase disk coercivity and strengthen disk signal amplitude, translating into superior archival data retention.
Oxygen (O)-containing composite PMR media can provide beneficial grain boundaries segregation by developing oxygen-rich grain boundary regions. Early granular media development work recognized the significant effect of oxygen (O) in suppressing the degradation of the anisotropy constant (“Ku”) resulting from thermal instability whenever the device underwent local overheating during operation. Oxygen (O)-containing media also exhibit low media noise and high thermal stability and are useful in high density PMR. Therefore, oxygen (O)-containing grain boundary regions in magnetic alloys acts as a grain refiner and a grain growth inhibitor, providing effective physical separation of grains. This physical separation, in turn, decreases grain-to-grain magnetic coupling, and increases SNR and thermal stability of the magnetization.
With conventional magnetic recording media, the magnetic data-storing layer 106 is deposited on top of a ruthenium (Ru)-based seed layer 105, where the purpose of seed layer 105 is to produce textured growth in the media layer. A high recording density, up to 200 Gbits/in2, is commonly achieved via the nucleation of nano-scale grains and effective grain isolation, allowing for a strong resistance to thermal agitation of the magnetization within the grains. Typically, in this mode of grain structure, a composite alloy such as (Co90Cr10)80Pt20-10SiO2 (mol. %) displays a grain magnetocrystalline anisotropy Ku value on the order of 7×106 erg. cm−3, which indicates a high thermal stability for this media.
Accordingly, it is desirable to provide cobalt (Co)-based alloy matrix compositions with a lower weight fraction of the ferromagnetic phase, and an appropriate volume fraction of the oxide constituent, in order to achieve the enhanced sputtering performances for the granular media sputtering material. In particular, it is desirable to provide a method for manufacturing oxide containing compositions which have a reduced amount of the total ferromagnetic phase, in order to minimize the amount of ferromagnetic phase in a sputtering target. In this regard, the present invention accomplishes these and other objectives by substituting the oxide or oxides of the most reactive element that would form upon sputtering by an oxide of the base matrix alloy as a source for oxygen, and introducing any reactive elements directly into the matrix in order to achieve a further degree of dilution of the matrix base metal.